Process for making composites of stretch broken aligned fibers and product thereof

ABSTRACT

A stretch broken fiber reinforced resin tow is formed from resin reinforced with continuous filaments by tensioning the tow while heating it to a temperature sufficient to soften the resin in the tensioning zone to break all of the filaments in a random manner.

This is a division of application Ser. No. 07/344,973, filed 4-26-89.

BACKGROUND OF THE INVENTION

This invention relates to a process for making a composite of a resinmatrix reinforced with stretch broken fibers and the product thereof.

U.S. Pat. No. 4,759,985, granted July 26, 1988, of common assigneediscloses a variety of methods for forming a composite of a resin matrixreinforced with stretch broken fibers such as winding a stretch brokensliver on a frame covered with a film of thermoplastic resin to form awarp. The warp of stretch-broken sliver, however, can be made by anytechnique known to those skilled in the art, e.g., by creeling orbeaming. A preform is obtained when another film of thermoplastic resinis placed over the warp to form a sandwich which is heated in a vacuumbag and then removed from the frame. Several of such preforms may bestacked while offset to provide multi-directionality and then the stackmay be heated under pressure to form a composite structure.

Other techniques for applying matrix polymer include sprinkling ofpowdered resin on the sliver warp followed by heating to melt the resin,flowing liquid resin over the sliver warp, intermingling thermoplasticfiber with the sliver warp and then heating to melt the thermoplasticfiber thereby forming the matrix resin, calendering the warp betweenlayers of matrix film, etc.

The composites formed are useful for deep drawing purposes with littlesacrifice of strength and stiffness as compared to composites formedfrom resin reinforced with continuous filaments. However, in U.S. Pat.No. 4,759,985, the fibers do translate considerable distances. Draftratios for this process typically run 200-300%. This high degree oftranslation creates the chance for alignment to be lost and a wavinessbecomes apparent on the surface of the tow. Finishes are applied to theyarn to reduce static, and to provide some cohesiveness to keepfilaments from flying away. But, finish acts only as a weak damper forthe energy released by the fiber when it breaks; much of this recoilenergy is eventually dissipated by fiber movement resulting inmisalignment.

SUMMARY OF THE INVENTION

The present invention provides an improved process for making acomposite of a resin matrix reinforced with stretch broken fibers andcomprises the steps of feeding a thermoplastic resin matrix towreinforced with continuous filament fibers into a tensioning zone thenheating the tow in the tensioning zone to soften the thermoplastic resinwhile tensioning the tow sufficiently to break all of the continuousfilament fibers in a random manner and finally cooling the tow.

Formable planar and shaped non-planar composites are contemplated by thepresent invention. For the formable composites, that is, thosecomposites that can be formed into shaped non-planar three-dimensionalstructures at elevated temperatures (where necessary), matrix resins ofthe thermoplastic variety may be employed. Suitable thermoplastic resinsinclude polyesters (including copolyesters), e.g., polyethyleneterephthalate, Kodar® PETG copolyester 6763 (Eastman Kodak); polyamides,e.g., nylon 6,6; polyolefins, e.g., polypropylene; also included are thehigh temperature resins such as an amorphous polyamide copolymer basedupon bis(para-aminocyclohexyl)methane, a semi-crystalline polyamidehomopolymer also based on bis(para-aminocyclohexyl)methane, andpolyetheretherketone. Fibers such as glass, carbon and aramid arepopular as reinforcement fibers.

The ratio of reinforcement fibers to matrix can vary, but preferably isbetween 40% to 75% by volume. The average fiber lengths also may varybut preferably range from about 1/2 to about 6 inches in length with arandom overlap distribution. About 95 percent of the fibers are alignedwithin ±5 degrees, and about 97 percent of the fibers are within ±10degrees of the axial direction.

In the present invention the high degree of alignment inherent in thefiber spinning process is preserved to a very high degree; it can evenbe enhanced by this process. The molten resin, which surrounds eachfilament at the time it is broken, acts as a damper to absorb the energyreleased by the fiber as it breaks. The resin catches the fiber, thisminimizes the conversion of recoil energy into energy which disorientsthe filaments. The energy is converted to heat by the resin and rapidlyconvected and radiated away to safety. The draft required to breakfibers using this process is much lower than for dry fibers. A draft of10% is sufficient to break fibers which have break elongations as highas 4%. In one sense, the process is very efficient since low draftsettings provide complete breakage of the tow bundle, i.e. each filamentbreaks. This results in part from the good grip that the breaker rollshave on the tow. This minimization of fiber movement reduces theopportunities for the filaments to become misaligned. The highlyviscous, molten thermoplastic resins, make excellent damping pots. Theprocess is also good because the resin is molten for such a short periodof time (<2 seconds). This limits the time in which the fibers may moverelative to one another. The tow is heated just before the breaking isto occur, and is cooled immediately afterward.

Because the fibers are surrounded by molten polymer during the stretchbreaking step, very little fly is generated. The tow is highly useful indownstream processing because the resin keeps the tow bundle contiguouseven under high tension. Thus, the tow can be woven into fabrics orfilament wound with effort similar to the processing of continuousfilament materials.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a schematic illustration of an apparatus for practicingthe process of the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

Referring to the drawing, the preferred embodiment generally includes acreel 10 holding a rotatable bobbin 12 of a tow 14 made of a resinmatrix reinforced with continuous filament fibers, a stretch breakingmachine 16 (Model 770 Rebreaker, Seydel, Bielefeld, Germany) with anintegral hot air heater 18 (6 KW shell-in-tube heater by Sylvania) and awindup 20 (Model 959 Leesona Corporation, Warwick, R.I.) for winding apackage 22. The stretch breaking machine 16 includes two breaker blockunits 22, 24. Unit 22 consists of driven roll 22a engaging and formingsuccessive nips with ceramic coated metal rolls 22b and 22c which arewater cooled. Roll 22a is covered with elastomer (Adiprene, L-325, 5/16"thick Shore D hardness of 75 applied by Standard Engineering,Wilmington, Del.). In a similar arrangement driven elastomer coveredroll 24a engages and forms nips with ceramic coated metal rolls 24b and24c. Roll 24a is covered with elastomer (54557 11/16" thick Shore Dhardness of 42 applied by Smoke Mobely, Washington, D.C.).

In operation the continuous filament fiber reinforced tow 14 is drawnfrom package 12 on creel 10 through guide 15 by means of driven roll 24ain conjunction with nip rolls 24b and 24c. The tow is pulled throughheater 18 by means o(driven roll 22a and associated nip rolls 22b and22c. Roll 22a is driven at a higher speed (about 10 percent faster) thanroll 24a to tension the tow. The conversion of the tow 14 into stretchbroken aligned fiber reinforced resin tow 14' occurs between rolls 22aand 24a. The tow 14 passes between the nips formed between rolls 24a,24b and 24c which grip the tow. The tow is then pulled through heater 18which softens the resin by raising its temperature to about its meltingpoint. Since the speed of roll 22a is faster than roll 24a a tension iscreated in the tow between the rolls which is sufficient to break eachof the continuous filaments in the tow between rolls 22a and 24a.Because the resin is soft the filaments do not transfer the shear loadthrough the resin to adjacent filaments and because no shear load istransferred the continuous filaments break randomly instead of all inone location. This random break distribution allows the tow 14' toremain continuous without separating. The resin cools rapidly afterleaving heater 18 and is rapidly cooled when moved over water cooledrolls 22b and 22c which are at a temperature of about 50° F. The stretchbroken tow is then wound into package 22 on winder 20 for furtherprocessing.

EXAMPLE I

One bobbin of 3700 denier continuous filament fiber reinforced resin towwas stretch broken using an apparatus as shown in the drawing. The towwas made up of two yarns of 1150 denier aramid yarn (Kevlar® 49 aramid,Du Pont) impregnated with PETG resin (Kodar® PETG copolyester 6763,Eastman Kodak). The resin includes 2% by weight carbon black (Ampacet).The tow is about 57% fiber on a volume basis.

The stretch break machine 16 was prepared by setting the center lines ofrolls 22c and 24b 7.25 inches apart. The surface speed of roll 24a wasadjusted to 11.8 meters/min and the surface speed of roll 22a was 13.0meters/min. Chilled water at about 40° F. was circulated through rolls22a and 22c. The right hand edge of the hot air gun nozzle was placed6.75 inches from the center line of roll 22c and its output temperaturewas about 650° F. Room temperature air was fed to the gun at the rate of10 cubic feet per minute. The stretch broken tow from stretch breakmachine is wound continuously into a package on the winder 20. The finaldenier of the tow is 3300.

A warp of the tow was prepared by wrapping the tow from its package, 2layers of 12 ends to the inch on a 18 inch square stainless steel plate1/16" thick. The entire plate was vacuum bagged in an oven for 1 hour at200° C. The product, called a prelam, was now a well impregnated,relatively stiff prelam of stretch broken fiber and resin with thefibers all aligned in one direction. The prelams were 12 mils thick.

Six prelams were cut to 5"×12" rectangles (fiber axes parallel to the12" sides). The prelams were stacked on top of each other in a mold. Themold was placed in a press and cured under 300 psi pressure per squareinch of composite at a temperature of 200° C. for 20 minutes. The presswas then cooled to 55° C. temperature and the 5"×12" composite rectanglewas removed. The rectangle was about 56 mils thick.

The rectangle was cut into pieces each 0.5" wide and 12" long. Thesample was sandblasted at 40 psi, 2 passes at both sides of both ends(last 17/8"). Aluminum tabs (1/8" thick, 9/16" wide, 2" long) were gluedto the sample (using Devcon "F" epoxy). The tabs and bars were placed ina tabbing frame and cured overnight. Nine samples were tested accordingto ASTM method D3039-76 entitled. "Standard Test Method for TensileProperties of Fiber-Resin Composites." The average tensile modulus ofthe samples was 10.4 million psi. The average tensile strength was 172thousand psi.

EXAMPLE II

One bobbin of 3580 denier continuous filament fiber reinforced resin towwas stretch broken using an apparatus as shown in the drawing. The towwas made up of two yarns of 1150 denier aramid yarn (Kevlar® 49 aramid,Du Pont) impregnated with resin (an amorphous polyamide copolymer basedon bis(para-aminocyclohexyl) methane). The tow is about 60% fiber on avolume basis.

The stretch break machine 16 was prepared by setting the center lines ofrolls 22c and 24b 8.0 inches apart. The surface speed of roll 24a wasadjusted to 5.5 meters/min and the surface speed of roll 22a was 6.0meters/min. Chilled water at about 40° F. was circulated through rolls22a, 22b and 22c. The right hand edge of the hot air gun nozzle wasplaced 6.75 inches from the center line of roll 22c and its outputtemperature was about 700° F. Room temperature air was fed to the gun atthe rate of 12 cubic feet per minute. The right hand edge of the gunnozzle is 6.75" from the center line of roll 22c. The stretch broken towfrom stretch break machine is wound continuously into a package on thewinder 20. The final denier of the tow is 3260.

A photomicrograph of the surface of the composite tow (enlarged 100×)was prepared. The tow samples were plasma etched at 50 Watt in 0.5 torrof Oxygen for 5 minutes. The samples were then coated with gold andphotographed using the JEOL 840 SEM at 100× magnification.

The orientation of the filaments was determined. After thephotomicrographs were obtained, the micrographs were photocopied toenhance the contrast between the filaments. The photocopy was then tapedto a digitizing pad (model DIGI-PAD5 made by GTCO Corporation, 1055First Street, Rockville, Md. 20850). The digitizing pad was connected toa PC (made by IBM). A program was created in the PC to accept andprocess the data. The orientation of each filament was entered into thecomputer by placing the crosshairs of the mouse (a moveable part of thedigitizing pad) over opposite ends of each filament in the photocopy(photocopies contained between 72 and 99 filaments each) and enteringthe location of each end by pressing a button on the mouse. Afterentering all the filaments on the photocopy, the PC was used to organizeeach data set, each photocopy, separately. The computer determined theangle of each filament relative to the digitizing pad by comparing thelocations of opposite ends of each filament. The angles were sorted fromthe most negative to the most positive. Then, the angles were"normalized" so that the mean angle was 0 degrees, to correct formisalignment in taping the pictures to the digitizing pad. Thisnormalizing was done by adding a constant to each angle. The number offilaments included within any angle was now determined by examining thedata and tabulating the results. This process could also be done byusing a protractor to measure the angles.

95.5 percent of the fibers were parallel to the axial direction of thetow within an angle of + or -5 degrees and 97.0 percent were within + or-10 degrees of the axial direction.

EXAMPLE III

One bobbin of 2670 denier continuous filament reinforced resin tow wasstretch broken using an apparatus as shown in the drawing. The tow wasmade up of one carbon fiber yarn (3K, AS-4W carbon fiber, Hercules)impregnated with resin (an amorphous polyamide copolymer based onbis(para-aminocyclohexyl) methane). The tow product is available from DuPont. The tow is about 65% fiber on a volume basis.

The stretch break machine 16 was prepared by setting the center lines ofrolls 22c and 24b 8 inches apart. The surface speed of roll 24a wasadjusted to 3.6 meters/min and the surface speed of roll 22a was 4.0meters/min. Chilled water at about 40° F. was circulated through rolls22a, 22b and 22c. The right hand edge of the hot air gun nozzle wasplaced 6.75 inches from the center line of roll 22c and its outputtemperature was about 600° F. Room temperature air was fed to the gun atthe rate of 12 cubic feet per minute. The stretch broken tow fromstretch break machine is wound continuously into a package on the winder20. The final denier of the tow is about 2400.

A photomicrograph of the surface of the composite tow (enlarged 100×)was prepared. The tow samples were plasma etched at 50 Watt in 0.5 torrof Oxygen for 15 minutes. The samples were then coated with gold andphotographed using the JEOL 840 SEM at 100× magnification usingsecondary electron imaging at 15 kV.

The orientation of the filaments was determined. After thephotomicrographs were obtained, the micrographs were photocopied toenhance the contrast between the filaments. The photocopy was then tapedinto a digitizing pad (model DIGI-PAD5 made by GTCO Corporation, 1055First Street, Rockville, Md. 20850). The digitizing pad was connected toa PC (made by IBM). A program was created in the PC to accept andprocess the data. The orientation of each filament was entered into thecomputer by placing the crosshairs of the mouse (a moveable part of thedigitizing pad) over opposite ends of each filament in the photocopy(photocopies contained between 43 and 59 filaments each) and enteringthe location of each end by pressing a button on the mouse. Afterentering all the filaments on the photocopy, the PC was used to organizeeach data set, each photocopy, separately. The computer determined theangle of each filament relative to the digitizing pad by comparing thelocations of opposite ends of each filament. The angles were sorted fromthe most negative to the most positive. Then, the angles were"normalized" so that the mean angle was 0 degrees, to correct formisalignment in taping the pictures to the digitizing pad. Thisnormalizing was done by adding a constant to each angle. The number offilaments included within any angle was now determined by examining thedata and tabulating the results. This process could also be done byusing a protractor to measure the angles.

92.8 percent of the fibers were parallel to the axial direction of thetow to within an angle of + or -5 degrees and 95.6 percent were within +or -10 degrees of the axial direction.

EXAMPLE IV

One bobbin of 9,840 denier continuous filament fiber reinforced resintow was stretch broken using an apparatus as shown in the drawing. Thetow was made up of one glass roving (E-glass, 6620 denier, #473CB675Type 30 roving, Owens Corning Fiberglass, 900 West Valley Road, Wayne,Pa. 19807) impregnated with resin (PETG, Kodar PETG copolyester 6763,Eastman Kodak). The resin includes 2% by weight carbon black (Ampacet).The tow is about 50% fiber by volume.

The stretch break machine 16 was prepared by setting the center lines ofrolls 22c and 24b 8.0 inches apart. The surface speed of roll 24a wasadjusted to 4.5 meters/min and the surface speed of roll 22a was 5.0meters/min. Chilled water at about 40° F. was circulated through rolls22a, 22b and 22c. The right hand edge of the hot air gun nozzle wasplaced 6.75 inches from the center line of roll 22c and its outputtemperature was about 700° F. Room temperature air was fed to the gun atthe rate of 12 cubic feet per minute. The stretch broken tow fromstretch break machine is wound continuously into a package on the winder20. The final denier of the tow is about 8860.

Photomicrographs of the surface of the composite tow (enlarged 100×)were prepared by photographing the surface of the tow through an opticalmicroscope. Twelve photomicrographs were prepared.

The orientation of the filaments was determined. After thephotomicrographs were obtained, the micrographs were photocopied toenhance the contrast between the filaments. The photocopy was then tapedonto a digitizing pad (model DP5A-1111A, made by GTCO Corporation, 1055First Street, Rockville, Md. 20850). The digitizing pad was connected toa PC (made by IBM). A program was created in the PC to accept andprocess the data. The orientation of each filament was entered into thecomputer by placing the crosshairs of the mouse (a movable part of thedigitizing pad) over opposite ends of each filament in the photocopy(photocopies contained between 34 and 68 filaments each) and enteringthe location of each end by pressing a button on the mouse. Afterentering all the filaments on the photocopy, the PC was used to organizeeach data set, each photocopy, separately. The computer determined theangle of each filament relative to the digitizing pad by comparing thelocations of opposite ends of each filament. The angles were sorted fromthe most negative to the most positive. Then, the angles were"normalized" so that the mean angle was 0 degrees to correct formisalignment in taping the pictures to the digitizing pad. Thisnormalizing was done by adding a constant to each angle. The number offilaments included within any angle was now determined by examining thedata and tabulating the results. This process could also be done byusing a protractor to measure the angles.

95.6 percent of the fibers were parallel to the axial direction of thetow to within an angle of + or -5 degrees and 96.1 percent were within +or -10 degrees of the axial direction.

We claim
 1. A tow of a matrix resin reinforced with substantiallyaxially aligned fibers broken in a random manner from a process of:feeding a thermoplastic resin matrix tow reinforced with continuousfilament fibers into a tensioning zone; heating said tow in saidtensioning zone to soften said thermoplastic resin while tensioning saidtow sufficiently to break all of said continuous filament fibers in arandom manner; and cooling said tow.
 2. The tow of claim 1 wherein saidcontinuous filament fibers are aramid fibers.
 3. The tow of claim 1wherein said continuous fibers are glass fibers.
 4. The tow of claim 1wherein said continuous filament fibers are carbon fibers.
 5. The tow ofclaim 1 wherein the ratio of reinforcement fibers to matrix resin isfrom about 40 to about 75% by volume.
 6. The tow of claim 1 wherein thebroken fiber lengths range from about 1/2 inch to about 6 inches.
 7. Thetow of claims 1, 2, 3, 4, 5 or 6 wherein 95% of said fibers are within±10 degrees of being axially aligned.
 8. The tow of claims 1, 2, 3, 4, 5or 6 wherein about 93% of said fibers are within ±5 degrees of beingaxially aligned.
 9. A compression molded article from the tow of claims1, 2, 3, 4, 5 or
 6. 10. A compression molded article from the tow ofclaim
 9. 11. A compression molded article from the tow of claim 8.